A range of lasers operating at various wavelengths can be used to inscribe or mark thermoplastic compositions with text, logos, and/or other identifiers. For example, opaque thermoplastic compositions can be marked with a 1064 nanometer (nm) laser light and rely on heat build-up in the substrate as the method of interaction between the laser light and the thermoplastic composition. Depending on the composition and/or the lasering parameters, the heat generated either causes a char, i.e., carbonization, to form a dark mark, or, alternatively a swelling from the formation of voids just below the surface and yields a light colored mark on a dark background, i.e., foaming. A vaporization process, or the bleaching of an additive, or a bleaching of the combination of additives, can also achieve light colored marks by exposing the lighter colored, thermally stable materials below the surface. The 1064 nm laser is also used to mark transparent compositions to give a dark mark. However, the inherent low levels of absorption by most thermoplastics in this area of the electromagnetic spectrum (i.e., 500 nm to 1,200 nm) can lead to inhomogeneous interactions between the laser light and the transparent substrate, which can lead to localized superheating and an inconsistent or poor quality mark. As a consequence, low levels of carbon black or other more specialized near infrared absorbing pigments or additives are often required in order to increase the quality and consistency of the mark. A disadvantage of using such additives is a reduction in visible transmission, increased haze and large color shifts due to the inherent residual color or particulate nature of the additives.
A photochemical interaction between the substrate and the laser light is another reaction mechanism by which identifiers are transferred to thermoplastics compositions. Lower wavelength light lasers such as those at 532 nm and especially with wavelengths lower than 400 nm (e.g., ultraviolet) generate contrast in polymer compositions in such a manner. The process is often referred to as “cold marking” due to the perception that there is limited thermal effect from the interaction of the lasers with the thermoplastic compositions. Ultraviolet lasers are predominantly used to mark opaque thermoplastic compositions containing titanium dioxide and yield a dark mark on white or light colored substrates.
However, there are some problems associated with the above processes. For example, the laser is not restricted to interacting at the surface or upper portion of the substrate to be marked and the intense light from the laser easily passes through the outer part or layer. In most cases, it is desirable that the mark is confined to the surface or close to the surface and that the laser beam does not interact or damage other materials or components beneath the layer or part to be inscribed. Exemplary designs or structures wherein the material to be marked houses devices or overlays other materials include a two shot or over-molded part such as in automotive glazing, a thermoplastic screen or housing encasing electronic devices such as mobile phones. One option is the addition of either IR absorbing additives or scattering additives, such as titanium dioxide, to the outer material, which restricts the transparency, haze, and colors available. Another problem is that an overt white or light colored mark is either impossible or the mark is a dark brown to tan color due to the persistent contribution from the carbonization effect and more particularly, in transparent compositions where a black mark is achieved due to strong carbonization of the material. Also, the durability of light colored marks is poor as the voids that cause the swelling are easily compressed and it is extremely difficult to achieve a semi-covert (i.e., barely visible) watermark, particularly in transparent compositions.
Another challenge exists when the material to be inscribed also functions as the laser transparent component in an article to be assembled by laser transmission welding (LTW). In general the lasers used for LTW are based on wavelengths longer than 800 nm. In order to join two components by LTW one component needs to be essentially laser transparent (e.g., no absorption), while the second component needs to absorb the laser light. The laser light then passes through the first component and heat is generated at the interface through the absorption of the laser light in the second component. The heat is conducted to the first layer, which melts and a weld is created upon re-solidification of both components. However, in some cases it is desired that the first component, which is transparent to laser light longer than 800 nm, also be inscribed with data. Even more desirable is the ability to inscribe a transparent, colored, opaque or dark colored first component suitable for LTW with a light mark. Thus, there is a need for the ability to generate a light colored mark on thermoplastic compositions.
It can also be desirable to be able to generate a range of contrast levels from an easily readable (e.g., light colored) mark to a semi-covert, barely visible, inscription with as little increase to the profile topography as possible, in order to eliminate the possibility of the layers lifting, separating, and/or cracking, for example when a laser inscription or mark is generated at the interface of two layers or at the interface of a thermoplastic layer and a coating. The change in the profile topography via carbonization, to form a dark mark or, alternatively, a swelling from a foaming process, can give profiles much larger than 50 micrometers and even larger than 100 micrometers. The generation of UV active or colored text, logos, barcodes or images, which are often incorporated into EID (“Electronic Identification”) cards passports, and pharmaceutical packaging requires the application of specialized inks and printing procedures. However, the specialized inks and printing procedures are costly and time consuming. Thus, a need exists to provide thermoplastic compositions with a customizable, machine-readable security function using a laser to generate microdots that exhibit either a different color under ultra-violet (UV) light source or are visibly of different color to the background.